A standard practice in modern day digital modules and computer systems is the use of printed circuit boards which are inserted and detached from a backplane in a supporting module. The printed circuit boards or digital cards will carry integrated circuit and other electronic resource elements which can, in total, often require large currents, often up to 50 amperes or more.
As long as the insertion of printed circuit boards into the backplanes is effectuated and the removal of printed circuit boards is effectuated during the times when no power is applied to the system, then there is no general problem in regard to the insertion and extraction of the printed circuit boards into the backplane connectors.
On the other hand, it is often not only desirable but necessary to remove and also reinsert printed circuit boards while the system is in a powered-up condition. Under these circumstances a certain number of problems and difficulties can arise during the course of insertion/extraction under power-up conditions.
When a printed circuit board or electronic card is inserted or extracted from a live system such as a backplane connector, a number of difficulties could arise. For example, power on the backplane could be disrupted thus bringing down the system. And then the problem of "rebooting" the system would take considerable time and effort to accomplish.
Additionally, viable data could be lost. Pins could be damaged from arcing current since the load would act as a short thus drawing a large surge of current through the first power pin to make contact. Also, the devices installed and located on the PC board or card could be damaged due to "latch-up" if the ground pin and the signal pin made contact before the voltage V.sub.cc. Thus a current surge shock could damage certain types of electronic circuits and freeze them so as to make them inoperable.
When the various pins of the backplane are caused to be connected and inserted into the female sockets of a PC Board Card, there is no guarantee that the various connections will take place simultaneously in time. Quite contrarily, there is no simultaneous-in-time connection which occurs and under these conditions certain conditions can occur which will cause damaging current surges and/or voltage glitches to occur in nearby circuits which can be conducive to error signals in these circuits.
Thus the extending fingers on the edge connector of a backplane motherboard will make various different points-in-time connections when a printed circuit card is plugged into the hot socket. Under these conditions the order in which the power and ground signals make contact will determine whether or not difficulties will be caused.
If the ground pin and the signal pin make contact before the power pin is connected, then integrated circuits connected to these fingers will take a voltage applied at their I/O pin before power is actually applied. This can cause what is described as a "latch up" in which there can occur a very low impedance path between different power supply lines. Currents may be generated to the point of even physical damage.
Another situation can occur when the power pins of a multiple-power supply system make contact before the ground pin makes contact. Under this situation many capacitors are often placed in series between the power pins, and since the capacitors are initially completely discharged, then the initial voltage at the ground will fall somewhere between the applied voltages depending upon the level of the capacitances. This results in a situation that the ground pin may be at a higher potential than the lower voltage power pin causing the cards (PC Boards) digital logic to be reversed.
Another type of unfortunate situation can occur when a cold unpowered printed circuit board, with its capacitors uncharged, is plugged into a hot socket backplane. The inrush of current from the power and ground traces will charge the capacitors and if no special circuitry is present to contain this current, then the uncharged capacitor will briefly cause a short between power and ground which causes a voltage glitch to appear on the backplane. This voltage glitch can be transmitted and appear at other cards plugged into the backplane near the card that is being inserted. This can corrupt data on other cards and even cause system failure.
One solution often suggested in these hot card insertion situations is that described in the magazine Electronic Design of May 11, 1989 in an article entitled "Insert Boards Into a Live System Without Any Hitches" located at pages 75 thru 80.
This article suggests that the printed circuit board's edge connectors can help solve some of these problems associated with hot-socket insertion by customizing different finger lengths of the edge connector so that the timing of various traces, grounds and signals can be controlled during insertion and extraction.
The present disclosure uses the concept of different length connective fingers in order to regulate the time of connection between the backplane and the PC Board and additionally provides specialized field effect transistor circuitry on the card which further prevents any problems occurring in the nature of shorted lines that might cause heavy currents or glitches which could corrupt signals in the vicinity.
Attempts in the prior art to partially handle the situation of insertion and extraction of cards in actively powered modules were very ineffective and cost consuming. Most of the earlier methods could not handle card loads greater than 35 milliamperes. Also, the early ideas of precharging the card capacitance would only work very limitedly for a very small load carrying card which consumed 35 milliamperes or, less but could not be used for cards drawing larger load currents.
The present system enables and it uses precharge pins, but not to attempt to precharge the capacitance of a high density current card such a 50 ampere card or PC board. In these cases, a series resistor would not be sufficient to control the loading effects.
The capacitive and DC loading effects of a card which draws 35ma to 50 amperes or greater requires a different approach to control the surges. The presently described circuitry takes into account all the possible capacitive and DC loading effects, uses only about 11/2 square inches of board space, consumes only about 2.5 milliamps for FET (field effect transistor) gate control and costs only a few dollars per board so as to handle up to 50 ampere load currents. Additionally, the present circuitry can be expanded or reduced to fit mode requirements by simply adding or subtracting the number of field effect transistors involved.